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Reaction of Zn+ with NO2. the Gas-Phase Thermochemistry Of

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Reaction of Zn+ with NO2. the Gas-Phase Thermochemistry Of Reaction of Zn+ with N02. The gas-phase thermochemistry of ZnO D. E. Clemmer, N. F. Dalleska, and P. B. Armentrouta) Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 (Received 24 Iune 199 1; accepted 2 August 199 1) The homolytic bond dissociation energies of ZnO and ZnO + have been determined by using guided ion-beam mass spectrometry to measure the kinetic-energy dependence of the endothermic reactions of Zn + with nitrogen dioxide. The data are interpreted to yield the bond energy for ZnO, D g = 1.61 t 0.04 eV, a value considerably lower than previous experimental values, but in much better agreement with theoretical calculations. We also obtain D g (ZnO + ) = 1.67 k 0.05 eV, in good agreement with previous results. Other thermochemistry derived in this study is D i (Zn +-NO) = 0.79 f 0.10 eV and the ionization energies, IE( ZnO) = 9.34 + 0.02 eV and IE( NO, ) = 9.57 + 0.04 eV. INTRODUCTION The measurement of neutral metal-ligand thermoche- mistry from the reactions of metal ions is accomplished by In a recent paper on the thermochemistry of gas-phase studying the endothermic transfer of a negatively charged transition-metal oxide neutral and ionic diatoms,’ we noted ligand to the metal, reaction ( I ) : that there is little information about the gas-phase thermo- chemistry of zinc oxide. Zinc is the only first-row transition M+ +XL-+ML+X+ (1) metal for which there have been no definitive measurements -rML+ +X. (2) made of the neutral oxide bond energy, D O(ZnO), and ioni- zation energy, IE( ZnO). In 1964, Anthrop and Searcy* at- This type of reaction has been used previously to obtain neu- tempted to study this molecule by high-temperature mass tral metal-hydride,8-‘s -methyl,9*‘0*‘2*‘6 and -oxide’*,“,‘* spectrometry, but did not detect “gaseous zinc oxide mole- bond energies. Since metal-containing species such as ML cules of any kind.” Based on estimates of their experimental have fairly low IEs, a competing process is reaction (2). sensitivity, they assigned an upper limit for the zinc oxide Successful observation of reaction ( 1) therefore requires bond energy of Di,, (ZnO) G2.86 eV. This same data was that the X fragment have a relatively low IE. The reevaluated in 1983 by Pedley and Marshall in their compi- Zn + + NO, system, where reaction ( 1) leads to formation lation on gaseous monoxides and they cite of ZnO+NO+, is chosen for the present study because DE ( ZnO) < 2.77 + 0.43 eV.3 In this same year, a lower lim- IE(N0) = 9.264 36 + 0.000 06 eV,19 much lower than the it of D ‘( ZnO) 22.8 + 0.2 eV was obtained by Wicke, who IEs of fragments of other small oxygen-containing mole- monitored the chemiluminescent reaction of zinc atoms cules such as 0, , CO,, alcohols, ketones, and aldehydes. with N, 0 as a function of kinetic energy.4 Wicke goes on to suggest that his lower limit, along with the upper limit of EXPERIMENTAL SECTION Anthrop and Searcy gives D ‘( ZnO) -2.8 eV. However, this General value is well above the theoretical values of D, = 1.20,0.66, Complete descriptions of the apparatus and experimen- and 1.44 eV calculated by Bauschlicher and Langhoff,s tal procedures are given elsewhere.20 Zn + production is de- Dolg et aZ.,6 and Igel-Mann and Stall,’ respectively. scribed below. The ions are extracted from the source, accel- Bauschlicher and Langhoff commented on this large dis- erated, and focused into a magnetic sector momentum crepancy but were unable to reconcile the difference. analyzer for mass analysis. Mass-selected ions are slowed to In this paper, we report the first direct measurement of a desired kinetic energy and focused into an octopole ion the bond energy and IE of the neutral ZnO molecule. This is guide that radially traps the ions. The octopole passes achieved by using guided ion-beam techniques to character- through a static gas cell containing the neutral reactant at ize the kinetic-energy dependence of the reaction of Zn+ pressures sufficiently low ( -0.05-0.13 mTorr) that multi- with NO,. In addition, we also measure the bond energy for ple ion-molecule collisions are improbable. After exiting the ionic zinc oxide, and compare this with a value previously gas cell, product and unreacted beam ions drift to the end of measured from reaction of Zn+ + O,, 0: (ZnO + ) the octopole where they are directed into a quadrupole mass = 1.65 f 0.12 eV.’ Further, since both the ionic and neu- filter for mass analysis and then detected. Ion intensities are tral zinc oxide molecules are formed in the present reaction converted to absolute cross sections as described previous- system, the energetic difference between these reaction path- ly.20 Absolute uncertainties in cross sections are generally ways allows a direct measurement of IE( ZnO). about + 20%. Relative uncertainties are much smaller. All product cross sections reported are the result of single ion- molecule collisions as verified by examining the pressure de- pendence of the product intensities. Laboratory ion energies are related to center-of-mass ” Camille and Henry Dreyfus Teacher-Scholar, 1987-1992. (CM) frame energiesby EC,, = Elabm/(M + m) whereM DownloadedJ. Chem. 28 JunPhys. 2002 95 (lo),to 129.79.63.125. 15 November 1991Redistribution 0021-9606/91/227263-06$03.00 subject to AIP license or copyright,@ 1991 see American http://ojps.aip.org/jcpo/jcpcr.jsp Institute of Physics 7263 7264 Clemmer, Dalleska, and Armentrout: Reaction of Zn+ with NO, and m are the ion and neutral reactant masses, respectively. o(E) ~00 Cgi(E-Eo +E, +Erot)“/Ey (3) Sharp features in the observed cross sections are broadened I by two effects: the thermal motion of the neutral gas, which which involves an explicit sum of the contributions of indi- has a width of -0.41E&$ for these reactions,*’ and the vidual reactant states, denoted by i, weighted by their popu- distribution of ion energies. The zero of the absolute energy lations, g,. Here, o, is a scaling factor, E is the relative kinet- scale and the ion energy distribution (full width at half maxi- ic energy, n is an adjustable parameter, and E, is the 0 K mumz0.4 eV lab) are measured by a retarding potential threshold for reaction of the lowest electronic level of the ion technique described elsewhere.*’ The uncertainty in the ab- with the lowest (O,O,O) vibrational level of NO,. E, repre- solute energy scale is + 0.05 eV lab. sents the vibrational levels of NO, populated at 305 K (the nominal temperature of the octopole) as calculated by a Ion sources Maxwell-Boltzmann distribution24*3’*32 and E,,, is the rota- Zn + used in these experiments has been produced by tional energy of the neutral reagent, which for room tem- two sources: electron-impact ionization of zinc metal vapor perature NO, is 3kT/2 = 0.039 eV. Before comparison with and argon-ion impact on a zinc metal surface. The electron- the experimental data, this model is convoluted with the impact source, described in detail previously,‘6 uses a resisti- neutral and ion kinetic-energy distributions as described vely heated oven to vaporize zinc powder. The resulting zinc previously.2o The mo, n, and E, parameters are then opti- atoms are ionized by electron impact at electron energies mized by using a nonlinear least-squares analysis to give the between 10 and 14 eV. Ionization of Zn with electrons hav- best reproduction of the data. Error limits for E. are calcu- ing energy less than 14 eV ensures that Zn + is produced in lated from the range of threshold values obtained for seven its electronic *S ground state since the ionization energy of different data sets with different values of n and the error in Zn is 9.394 eV and the first excited state of Zn + lies 6.01 eV the absolute energy scale.33 higher in energy. ** The second source is a flow-tube In order to model some data channels, we also use a source23 that uses a dc discharge24 in a 10% Ar in He flow to modified form of Eq. (3) which accounts for a decline in the sputter a zinc metal surface and create zinc ions. Both product ion cross section at higher kinetic energies. This sources yield similar results indicating that only ground- model has been described in detail previously,34 and de- state Zn + (*S) is formed. pends on ED, the energy at which a dissociation channel or a competing reaction can begin, andp, a parameter similar ton NO, purification in Eq. (3). Nitrogen dioxide as obtained from Matheson in 99.5% purity was subjected to multiple freeze-pump-thaw cycles RESULTS using liquid nitrogen. During the first freeze cycle, the solid Four products, formed in reactions (4)-( 7), phase (N, 0, > was always a blue-green color indicating that N, 0, was also present. Before thawing, an excess of oxygen Zn + (*S) + NO, NO,C + Zn (4) gas (roughly 10-20 times more than the initial NO, added) ZnO+ + NO (5) was introduced to the system. The frozen nitrogen oxides were allowed to warm up in the excess of oxygen such that NO+ + ZnO (6) any NO formed from decomposition of N,O, reacted with 1 ZnNO + + 0, oxygen to form NO,. Upon refreezing the resulting NO, (7) and oxygen mixture, the resulting solid phase was always a are observed from the reaction of Zn + (*S) with NO,.
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